GLOSSARY OF PARAMETERS

 Carbon Monoxide (CO) – Carbon monoxide air pollution is produced from the partial oxidation of carbon-containing compounds, notably in internal-combustion engines. Carbon monoxide forms in preference to the more usual carbon dioxide when there is a reduced availability of oxygen present during the combustion process. It is a colorless and odorless, tasteless, yet highly toxic gas. Both state and federal ambient air quality standards have been established for carbon monoxide. These standards pertain to one-hour and eight-hour averages. The federal primary standards are 35 parts per million (ppm) for a one-hour averaging time and 9 ppm for an eight-hour averaging time. The Hawaii standards are more stringent and are set at 9 ppm for a one-hour averaging time and 4.4 ppm for an eight-hour averaging time. The allowable one-hour and eight-hour concentrations may not be exceeded more than once per year. The West Oahu Air website shows the measured one-hour and eight-hour concentrations of carbon monoxide in units of parts per million. Air Quality Index values are calculated and displayed for the eight-hour averaging time only.

Nitric Oxide (NO) – Nitric oxide is a toxic air pollutant produced by cigarette smoke, automobile engines, fuel-fired power plants (fossil and or bio fuels) and other industrial sources. When emitted into the atmosphere, it generally rapidly converts to other chemical forms, especially in the presence of ozone. Although nitric oxide is measured at the West Oahu Air Monitoring Stations and the one-hour averages are reported on the website, there is no specific state or federal ambient air quality standard for NO. Measurements of NO are primarily used to determine levels of nitrogen dioxide (NO2), for which there are state and federal ambient air quality standards. The West Oahu Air website shows the measured one-hour average concentrations in units of parts per billion (ppb). Because there is no specific ambient air quality standard for nitric oxide, an Air Quality Index value is not calculated for this parameter.

Nitrogen Dioxide (NO2) – Nitrogen dioxide is a reddish-brown toxic gas that has a sharp, biting odor and is a prominent air pollutant. It is generally not emitted directly from industrial sources but forms in the atmosphere as a result of emissions of nitric oxide that undergoes chemical transformations in the presence of oxygen or ozone. Nitrogen dioxide is not measured directly at the West Oahu Air Monitoring Stations. Rather it is measured as the difference between the measured concentrations of nitric oxide and oxides of nitrogen (NOx). There are both Hawaii and federal standards for nitrogen dioxide. The allowable concentration for an annual averaging period is set at a concentration level of 37 parts per billion (ppb) by Hawaii, while the federal annual primary and secondary standard is 53 ppb. Until recently, there were no short-term standards for nitrogen dioxide. In January 2010, the U.S. EPA published a new one-hour primary standard for nitrogen dioxide at a level of 100 ppb. Compliance is determined based on a three-year average of the annual 98th-percentile value of the daily one-hour maximum. Information shown on the West Oahu Air website reflects the new standard and the corresponding Air Quality Index.

 Oxides of Nitrogen (NOx) – The term oxides of nitrogen (NOx) is the term given to the total concentration of nitric oxide (NO) plus nitrogen dioxide (NO2). The same instrument is used to measure three related parameters: nitric oxide, nitrogen dioxide and oxides of nitrogen, and the measured concentrations are reported in units of parts per billion (ppb). There are no specific ambient air quality standards for oxides of nitrogen, but it is used to determine the nitrogen dioxide concentration (for which there are state and federal standards). After the nitric oxide concentration is subtracted from the oxides of nitrogen concentration, the remainder is nitrogen dioxide. Because there is no specific ambient air quality standard for oxides of nitrogen, an Air Quality Index value is not calculated for this parameter, but the West Oahu Air website does report the one-hour average concentration of oxides of nitrogen in units of ppb.

 Ozone (O3) – Ozone is generally not emitted directly by air pollution sources but instead is produced by chemical transformations that occur in the atmosphere between oxides of nitrogen and oxygen in the presence of sunlight. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of humans and animals. However, the ozone layer in the upper atmosphere filters potentially damaging ultraviolet light from reaching the earth's surface. Thus, ground-level ozone can be detrimental to health, whereas upper-level ozone is essential for protection from the sun's harmful radiation. Ozone is present in low concentrations throughout the Earth's atmosphere. Hawaii has relatively high natural background levels of ozone due to the abundance of sun and because it is surrounded by water (which reduces natural ozone destruction). Both state and federal ambient air quality standards have been established for ground-level ozone. Presently, both the federal primary and secondary standards limit the eight-hour average ozone concentration to 70 parts per billion (ppb), while the Hawaii ozone eight-hour standard is 80 ppb. The eight-hour average is computed as a moving average; the three-year average of the fourth highest daily maximum value may not exceed the level of the standard. The West Oahu Air website shows the measured one-hour and eight-hour concentrations of ozone in units of parts per billion. Air Quality Index values are calculated and displayed for the eight-hour averaging time only.

 Particulate Matter (PM) – Particulate matter air pollution is produced by both natural sources (such as wind blown dust, sea salt, pollen, etc.) and anthropogenic sources (such as fuel combustion, rock quarrying/crushing and agriculture). High levels of particulate matter in the atmosphere are associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death. Larger particle sizes are generally considered to be mostly a nuisance. It is the smaller particle sizes, generally less than 10 microns, that are considered the most damaging to health. Both state and federal ambient air quality standards have been established for particulate matter. These standards pertain to 24-hour and annual averaging times and to two different size fractions: less than 10 microns and less than 2.5 microns. Both the 10-micron size fraction (PM-10) and the 2.5-micron size fraction (PM-2.5) are measured at the West Oahu Air stations.  Both the Hawaiiand the federal standards are set at the same concentration levels for PM-10 for a 24-hour average: 150 micrograms per cubic meter.  There is also an annual Hawaii PM-10 standard set at 50 micrograms per cubic meter. The allowable 24-hour PM-10 concentration may not be exceeded more than once per year. The federal primary and secondary 24-hour standards for PM-2.5 are both set at 35 micrograms per cubic meter, while the annual primary standard is 12 micrograms per cubic meter and the annual secondary standard is 15 micrograms per cubic meter. The federal 24-hour PM-2.5 standards pertain to the 3-year average of the 98th percentile 24-hour concentrations. The West Oahu Air website shows the measured one-hour and 24-hour concentrations of PM-10 and PM-2.5 in units of micrograms per cubic meter. Air Quality Index values are calculated and displayed for the 24-hour averaging time.

Precipitation – Precipitation is measured at the West Oahu Air stations by a tipping bucket rain gauge mounted on the roof of the station at a height of about 10 feet above ground. The precipitation reported on the West Oahu Air website pertains to the hourly total and is reported in units of inches. The 24-hour total is also indicated and reported in units of inches.

Sulfur Dioxide (SO2) – Sulfur dioxide is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, which can result in acid rain. Sulfur dioxide is an allergen to which some people are sensitive. SO2 is associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death. Both state and federal ambient air quality standards have been established for sulfur dioxide. In June 2010, the U.S. EPA published a new primary one-hour standard for SO2 and revoked the 24-hour and annual primary standards. The new federal one-hour standard for SO2 is set at 75 ppb. Compliance is determined based on a three-year average of the annual 99th-percentile value of the daily one-hour maximum. This revises the previous standards of 140 ppb over 24 hours (second-highest annual value) and 30 ppb evaluated over one year. To date, Hawaii standards remain set at the previous federal levels, including a three-hour average of 0.5 ppm. The West Oahu Air website displays Air Quality Index values corresponding to the new one-hour federal standard. Three-hour and 24-hour average concentrations will also be reported (but AQI values for these averaging periods will not be calculated).

Temperature – Temperature is measured at the West Oahu Air stations at a height of approximately 30 feet above ground. The temperature reported on the West Oahu Air website pertains to the hourly average and is reported in units of degrees Fahrenheit (deg F).

Wind Direction – Wind direction is measured at the West Oahu Air stations at the standard height of 33 feet above ground by a wind vane mounted on a tower. The wind direction reported on the West Oahu Air website pertains to the hourly average and is reported in terms of the direction sector from which the wind is blowing. A blowing wind from the north is indicated as "N", a wind from the northeast is indicated as "NE", etc.

Wind Speed – Wind speed is measured at the West Oahu Air stations at the standard height of 33 feet above ground by a propeller anemometer mounted on a tower. The wind speed reported on the West Oahu Air website pertains to the hourly average and is reported in units of miles per hour (mph).